I would have thought there was a setting for this. I even looked in the 'about:config' section for a prefix character, but I couldn't see anything that matched. That's not say it isn't there though.

When I forward/reply to an email, I get blue lines running down the left and right sides of the message, but that is because I use HTML formatting as the default setting for email composition. If I switch to TEXT formatting, I see your issue.

Maybe HTML formatting for email composition is the way forward?

If > doesn't appear elsewhere in the message, could you use Replace (Ctrl+h) to change "> " (greater than symbol followed by space) to nothing?

Try the QuoteAndComposeManager add-on (see picture). To install the add-on, drag the Download link for the xpi file and drop it onto Tools/Add-ons/Extensions in TB.

sfhowes said

QuoteAndComposeManager

That's looks pretty good. I, personally, didn't see this as a problem as yet, but that solution is worth looking at anyway.